Function drag_need 

pub(crate) fn drag_need(
    qt: &CxxQtThread<Sioul>,
    shared: &Arc<Shared>,
    edit: &str,
) -> String

A day’s block moved by a drag on the page’s timeline (change_need’s JSON: “move”, or “times” for one of its ends), changed as “Move to…” and “Change its times…” change it, then “Undo” offered for ten seconds, which puts that day’s block back as it was. Dropped where it was: nothing changes, nothing to undo. Returns what went wrong, else “”.